Airbags were developed to protect occupants of motor vehicles during crashes. Originally airbags were viewed as an alternative to seat belts, because they are passive devices which do not require the cooperation of the vehicle's occupants to be effective. However, experience has shown that airbags should be considered an adjunct to seat belts. When used properly, airbags can in certain crash situations reduce injuries to vehicle occupants. An airbag is a gas filled bag which is rapidly deployed in the event of a vehicle crash. When appropriate sensors determine that a crash situation has occurred, the airbag is rapidly inflated to create an impact cushion between the occupant(s) of a vehicle and the structure of the vehicle.
An airbag system comprises two basic functional components: the airbag, including the means for deploying it; and the sensors and circuitry which determine whether the airbag should be deployed.
Recently, sensors have been developed to determine the weight imposed on the front seat, and the value of the determined weight may then be used as an input into the system logic which determines whether or not an airbag should be deployed, or whether the mode of deployment should be modified. In some cases, these weight sensors can be confused by loads imposed by the compression of the seat resulting from installing a child's car seat and tensioning the seat belt to hold the child seat into place. Another area of concern as a factor to be considered by airbag deployment logic is whether the occupant is properly wearing a seat belt.
Sensors to determine if a seat belt is buckled were first widely used to remind the occupants of a motor vehicle to fasten their seat belts. Recently, airbags have been used in automobiles to enhance and complement the protection provided by seat belts. This combination of seat belts and airbags can be optimized if it can be determined that the seat belt is actually in use. If the seat belt is fastened, then deployment logic can be based on knowledge of that condition. Accurate sensing information of seat-belt latch status is far more important when used as part of the decision logic and deployment strategy of an airbag system then when used simply as part of a seat belt fastening reminder system.
The typical improved seat belt latch indicating system such as might be used in combination with an airbag deployment system, is a mechanical switch within the buckle of a seat belt combined with a resistor network. The simplest resistor network has a first resistor connected in series with the switch and a second resistor connected in parallel with the switch. Thus the resistance of the switch and the resistor network changes when the switch is closed. But the switch, because of the resistor connected in parallel across the open leads of the switch, can be positively monitored when open. Thus if resistance falls to zero or has a resistance value substantially different from the two designed states of the switch and resistor network, it is clear that the seat belt latch detector is broken.
The environment in which seat belts are installed can be a hostile one. The switch is called upon to operate when dirty, wet, or even when coated with residue from spilled soft drinks. Further, the use of seat belt latch sensors to sound reminder chimes or a buzzer is sometimes perceived as annoying, and can lead those resistant to wearing safety belts to disable a seat belt latch indicator by causing the buckle to latch without engaging the hasps of the belt. Where seat belt latching information is being used in an airbag deployment logic, the folly of disabling safety equipment can prevent the deployment logic from making the best possible decision by providing false information to the logic.
What is needed is a seat belt sensor which can monitor latch status and belt tension, and which is both durable and better able to monitor latch status.